To promote the safety of aircraft occupants, aircraft must meet certain head injury criteria (HIC) requirements. To meet HIC requirements, the head movements of an aircraft occupant in an HIC event should result in a minimal or below-threshold head injury. For example, aircraft certification in the United States requires engineers to demonstrate that a head strike into any one of several cabin furnishings complies with the HIC requirements specified in 14 C.F.R § 23.562 and 14 C.F.R § 25.562. HIC requirements may be satisfied in several ways, such as by providing head clearance and/or soft impact surfaces around the aircraft occupant. Both of these solutions, however, add cost and weight to the aircraft.
Aircraft must include restraint systems to enhance occupant safety. For example, some aircraft include fixed seatbelts, which have a strap that does not extend in response to the bodily movement of the occupant. While fixed seatbelts may provide a short HIC trajectory, allowing for smaller seat pitch installation, fixed seatbelts are uncomfortable and may severely limit the mobility of the occupant, including hampering the pilot's ability to reach controls and look through windows. Other aircraft include inertia reel seatbelts, which have a strap that is extendable or retractable except when occupant movement reaches a threshold that causes the inertia reel to lock, preventing further extension of the strap. Inertia reel seatbelts have a delayed reaction time for locking the occupant into a secured position, resulting in increased head motion than that allowed by a fixed seatbelt. Accordingly, a need has arisen for a modal restraint system that provides a similar or superior range of head motion as compared to a fixed seatbelt, while also increasing occupant comfort and mobility.